Vehicle steering is generally controlled by a driver hand wheel that directs the angle of the vehicle road wheels used for steering. The movements of the driver hand wheel are transmitted to the vehicle road wheels by mechanical linkages and/or electronic components. The vehicle road wheels that change angle are generally located in the front of the vehicle in a system referred to as “front steering”. The angle of the road wheels is referred to as road wheel angle.
Active front steering (AFS) is a term referring to the use of electronic components to actively control or assist the steering of a vehicle so as to enhance steering performance beyond that possible by only direct mechanical linkages. There are many possible ways to enhance steering performance; for example, steering can be adapted to the weather conditions, to the behavior and habits of the driver, provide orderly stopping if the driver loses control, enhance the driver hand wheel control by changing steering characteristics, or provide driver control in the event of a steering mechanism malfunction.
In an AFS system, variable gear ratio (VGR) steering is a method for adding and subtracting steering angle to or from the target road wheel angle implied by the driver's hand wheel input. This can be accomplished by mechanical, electrical components, and combination devices like actuator motors. It is desirable to insure that the VGR system is fail-safe, operates in a safe manner, and does not vary greatly from its intended operational parameters.
In an AFS system, the intended angle at the hand wheel and the actual angle at the front steering wheels are monitored by sensors; generally Hall effect sensors. A Hall effect sensor is an electronic device that varies its output voltage in response to changes in magnetic field density. When a magnetic field is perpendicular to the surface of a sheet of conductive material, an electric field is created across the surface. For a given magnetic field, the distance from the magnet to the sheet can be determined. Using groups of sensors, the relative position of a known magnet can be determined. By measuring relative position, Hall effect sensors can be used to time the speed and position of wheels and control shafts. Due to their magnetic nature Hall effect sensors are non-contacting so they don't have wear from contact over time. Because they do not require direct contact, Hall effect sensors are generally not affected by dust, dirt, mud, water, and oil so they are ideal for the dirty environment of automotive applications. A Hall effect sensor may have circuitry that allows the device to act in a high/low voltage switch mode. Other binary devices that allow the sensors to act in a high/low voltage switch mode may also be used to time the speed and position of the wheels and the control shafts, including, without limitation, transistors. Hall effect sensors are generally located in the spiral cable at the magnet plate between the wave motion generator and the flexible gear in an AFS system.
In a typical AFS system, there are three sensors that are used to measure the angle of the steering actuator. Each sensor is either in a “high” state (for example, corresponding to a 12 volt output) or a “low” state (for example, corresponding to a 0 volt output). The functional status of the three sensors is determined by a diagnostic system. The diagnostic system may include measuring the output angle indicated by the sensor in response to predetermined actuator motor angle positions.
An AFS system may have errors or malfunctions in the actuator angle sensors or in the AFS system's processors. Processor errors may occur due to electrical or other faults. The processor may continue to have some functionality after certain types of errors or faults have been detected. At the extreme, some processor errors may result in a significant loss of functionality. For this reason, some AFS systems utilize two redundant processors as a fail-safe measure.
At present, if there is an error in the processors, implementing the diagnostic algorithm, it is not coordinated between the processors. If there is a processor error, then there could be false detection of an AFS system error (alpha error) or non-detection of a defective AFS system (beta error). Any system or method to detect processor errors should also not introduce its own additional alpha and beta errors.
When an error is detected in a particular sensor by an AFS diagnostic system processor, the actuator motor may be mechanically locked to prevent potential damage to the variable gear steering and other AFS system components including an actuator motor lock holder. Mechanical locking involves setting a fixed steering ratio with a physical device. At present, both the main and the sub-processor can mechanically lock the actuator motor by removing power to the actuator motor, and the actuator motor may be electrically phase-locked before mechanically locking. At present, the AFS system has the ability to electrically lock the AFS actuator motor only via the main processor. Moreover, if an error occurs in the operation of an AFS system control processor, the sub-processor is only capable of locking the actuators mechanically and not electrically.
It is desirable to design a new system and method to allow the AFS system sub-processor to lock the actuator motor electrically if an error occurs. Additionally, it is desirable to have an AFS system diagnostic technique that is robust to false failures and allows the AFS system to safely go to a fail-safe mode if an error occurs. Other desirable features and characteristics of embodiments of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.